Esters have been used as lubricating oils for over 50 years. They are used in a variety of applications ranging from jet engines to refrigeration. In fact, esters were the first synthetic crankcase motor oils in automotive applications. However, esters gave way to polyalphaolefins (PAOs) due to the lower cost of PAOs and their formulation similarities to mineral oils. In fully synthetic motor oils, however, esters are almost always used in combination with PAOs to balance the effect on seals additive solubility, volatility reduction, and energy efficiency improvement by enhanced lubricity.
Ester-based lubricants, in general, have excellent lubrication properties due to the polarity of the ester molecules of which they are comprised. The polar ester groups of such molecules adhere to metal surfaces creating protective films which slow down the wear and tear of the metal surfaces. Such lubricants are less volatile than the traditional lubricants and tend to have much higher flash points and much lower vapor pressures. Ester lubricants are excellent solvents and dispersants, and can readily solvate and disperse the degradation by-products of oils. Therefore, they greatly reduce sludge buildup. While ester lubricants are stable to thermal and oxidative processes, the ester functionalities give microbes a handle with which to do their biodegrading more efficiently and more effectively than their mineral oil-based analogues—thereby rendering them more environmentally-friendly. However, the preparation of esters is more involved and more costly than the preparation of their PAO counterparts.
Recently, novel diester-based lubricant compositions and their corresponding syntheses have been described in commonly-assigned U.S. patent application Ser. No. 11/673,879; filed Feb. 12, 2007. The synthetic routes described in this patent application comprise and/or generally proceed through the following sequence of reaction steps: (1) epoxidation of an olefin to form an epoxide; (2) conversion of the epoxide to form a diol; and (3) esterification of the diol to form a diester.
More recently, additionally novel syntheses of the above-mentioned diester-based lubricant compositions have been described in commonly-assigned U.S. patent application Ser. No. 12/244,349; filed Jan. 31, 2008. Such syntheses are similar to those described above, but do not proceed through a diol intermediate. Rather, they involve a direct esterification of the epoxide.
In view of the foregoing, and not withstanding such above-described advances in diester-based lubricant synthesis, any variation on such above-described methods that can reduce the concentration of the epoxidizing agent(s), increase yields, and/or minimize purification requirements, would be highly beneficial.